1. Field of the Invention
The present invention relates to a composite particle for an electrode, which becomes a constituent material of an electrode usable in an electrochemical device such as a primary battery, a secondary battery (lithium ion secondary battery in particular), an electrolytic cell, and a capacitor (electrochemical capacitor in particular), an electrode and electrochemical device using the same, and methods of making them.
2. Related Background Art
Electrochemical devices such as electrochemical capacitors represented by electric double layer capacitors and batteries represented by lithium ion secondary batteries are easy to reduce their size and weight, and thus are expected to become backup power supplies for power supplies of portable devices (small-size electronic devices) and auxiliary power supplies for hybrid cars, for example.
An electrochemical device is mainly constituted by a pair of electrodes, and an electrolyte layer (e.g., a layer constituted by an electrolytic solution or solid electrolyte). Conventionally, the electrodes are made by the steps of preparing a coating liquid (e.g., in a slurry or paste form) containing an electrode active material (active carbon or the like), a binder (synthetic resin or the like), a conductive auxiliary agent, a dispersant and/or solvent; applying the coating liquid to a surface of a collector (e.g., metal foil); and drying the coating liquid, so as to form a layer containing the electrode active material (hereinafter referred to as “active material containing layer”) on the surface of the collector.
In this method (wet method), there is a case where no conductive auxiliary agent is added to the coating liquid. There is also a case (dry method) where, without using the dispersant and solvent, a kneaded product containing the electrode active material, binder, and conductive auxiliary agent is prepared in place of the coating liquid, and this kneaded product is shaped into a sheet by using a heated roller and/or hot press. There is a case where a conductive polymer is further added to the coating liquid, so as to form so-called “polymer electrode”. When the electrolyte layer is solid, there is a case where a method including the process of applying the coating liquid to the surface of the electrolyte layer is employed.
The above-mentioned electrochemical device has been under study and development in order to achieve further improved device characteristics (e.g., higher capacity, improved safety, and higher energy density) in conformity to developments in portable devices from now on. In particular, electrochemical capacitors such as electric double layer capacitors have been hard to achieve a further improvement in electrostatic capacity, since there is a limit to the capacity of active carbon or the like used as the electrode active material.
In the field of batteries, for example, a method of making a positive electrode mixture for an organic electrolyte battery comprising the steps of preparing a slurry comprising a positive electrode material (cathode active material), a conductive agent (conductive auxiliary agent), a binder, and a solvent, the slurry having a solid content of 20 to 50 wt % with an average particle size of 10 μm or less; and granulating the slurry by spray drying has been proposed for further improving characteristics such as discharge characteristic and productivity (see, for example, Japanese Patent Application Laid-Open No. 2000-40504).